Gyroscopes (also referred to herein as gyros) have been used to measure rotation rates or changes in angular velocity about an axis of rotation. A basic conventional fiber-optic gyro (FOG) includes a light source, a beam-generating device, and a coil of optical fiber coupled to the beam generating device that encircles an area. The beam-generating device transmits light beams into the coil that propagate in a clockwise (CW) direction and a counter-clockwise (CCW) direction along the core of the optical fiber. Many FOGs utilize glass-based optical fibers that conduct light along a solid glass core of the fiber. The two counter-propagating (e.g., CW and CCW) beams experience different path lengths while propagating around a rotating closed optical path, and the difference in the two path lengths is proportional to the rotational rate that is normal to the enclosed area.
In a conventional resonator FOG (RFOG), the counter-propagating light beams are typically monochromatic (e.g., in a single frequency) and circulate through multiple turns of the fiber-optic coil and for multiple passes through the coil using a device, such as a fiber coupler, that redirects light that has passed through the coil back into the coil again. The beam-generating device modulates and/or shifts the frequencies of each of the counter-propagating light beams so that the resonance frequencies of the resonant coil may be observed. The resonance frequencies for each of the CW and CCW paths through the coil are based on a constructive interference condition such that all light-waves having traversed the coil a different number of times interfere constructively at any point in the coil. As a result of this constructive interference, an optical wave having a wavelength λ is referred to as “on resonance” when the round trip resonator optical path length is equal to an integral number of wavelengths. A rotation about the axis of the coil produces a different effective path length for clockwise and counterclockwise propagation, thus producing a shift between the respective resonance frequencies of the resonator. The frequency difference, such as may be measured by tuning the CW beam and CCW beam frequencies to match the resonance frequency shift of the closed optical path due to rotation, indicates the rotation rate.
However, the modulation of the light beam also has imperfections that can result in rotation rate bias errors. One type of imperfection is modulator intensity modulation. Even though the intended modulation is either on cavity length, optical frequency or optical phase, a non-ideal modulator may also generate a modulation of the light intensity which can have a component at the modulation frequency. The unwanted intensity modulation will be detected by the demodulator and interpreted as a signal indicating an off resonance condition. Resonance tracking electronics will then move the laser frequency away from the resonance frequency until the normal resonator intensity signal exactly cancels out the unwanted intensity signal. The deviation away from the resonance frequency results in a rotation sensing error if the unwanted intensity signals are different between the two counter-propagating light waves.
Another modulator imperfection that can result in rotation-sensing errors is modulation distortion. Modulation distortion can occur at the modulator drive electronics or the modulator. An ideal modulation is a sinusoidal modulation at a single frequency. However, distortion can result in the generation of higher harmonics on the modulation. Even harmonic modulation will result in a resonance detection error which can lead to a rotation sensing error.
Another source of rotation sensing error is laser phase noise occurring at frequencies above the resonance tracking modulation frequencies. Laser phase noise at twice the resonance tracking modulation frequency and other higher frequency bands will be converted to relative intensity noise by the gyro resonator and frequency down-converted to the same frequency as the resonance detection signal, and therefore eventually becomes a random rotation sensing error. Some methods to reduce this type of error to acceptable levels are to employ lasers with very low phase noise and optical filtering. But this can lead to significant cost increase of the product and reduce product operational robustness.
If the phase modulation is applied to both beams in common, then the modulation imperfections are commonly shared between the CW and CCW light beams and the modulation imperfections can be subtracted out during signal processing of gyro rate signals. However, common modulation can result in other rate bias errors that are associated with optical backscattering. Two techniques for suppressing optical backscattering include using intensity modulation to encode the various laser beams with a unique signature and balanced heterodyne detection at the resonator output to allow the separation of various detected beams in the signal processing. However, the use of intensity modulation includes intensity modulators that introduce significant optical losses that result in a degradation in angle random walk performance. Also, the use of balanced heterodyne detection includes extra optics and complex signal processing and control.
Another benefit of common resonance tracking modulation is that the random error generated by common laser phase noise at frequencies above the modulation frequency will subtract out during signal processing of gyro rate signals. The rejection of common mode laser phase noise can allow the use of lower cost, higher phase noise lasers and can eliminate the need for optical filtering, which can result in a significant reduction in product cost and greatly improve the product's operational robustness.